104 THE AMERICAN MONTHLY [June, 
of the part of the cell which directly faces the cavity. The substance within 
the cell may be traced to the outer cell-wall, but never beyond it. The part 
of the cell which lies next this wall we will call the outer end of the cell, as 
the part next the basement membrane is the inner end of the cell. The dis- 
tance between the inner and outer cell-walls is the height of the cells here, 
__! _ of an inch in a few cases. 
1000 ; 
e. The side walls of the cell are often very hard to see, hence the exact 
contour of the cell is very often almost impossible to determine. It is easy 
to see why this is so. The animal cell wall is so extremely thin that we 
scarcely ever see it when it lies in contact with the contents of the cell, espec- 
ially if this substance is placed on both sides of it by the contact of two cells. 
Now the end walls stand between very unlike substances, and their position, 
if not the actual walls, is easily determined, while the side walls, wedged 
between two very similar masses of protoplasm, are often almost or quite 
invisible. They can, however, be seen in various places if one is patient, and 
often careful focussing will bring them to view. When they can be seen they 
give us, of course, the exact boundary of the cell as it was left by the various 
preservative reagents. They can never be seen in every part of a section as 
plainly as it is the custom to represent them in illustrations, and in the fig- 
ures (plate iv, fig. 2) illustrating this paper they are drawn too plain, for the 
purpose of making it possible to trace their outline. The strip on the extreme 
right of that figure is, however, faithful to the section represented. In tracing 
the outline of the cell the observer meets very great difficulty from this faint- 
ness of the cell-wall, and it is, perhaps, the most dissatisfying experience at 
first in the attempt to determine the boundaries of cells such as these. 
jf. The protoplasm.—While the boundary of the cell is thus very obscure, 
the protoplasmic substance within the cell, by reason of its affinity for the 
staining fluid, is always brought into prominence. This is differently dis- 
ributed in different kinds of cells, or even different similar cells of a single 
organ. In the part at present under consideration (see fig. 3) there is a very 
marked difference between the inner and outer ends of the cells. The outer 
ends are very transparent, and faintly stained, with only here and there an 
indication of granular matter. Somewhat deeper the tint becomes deeper 
and the granules more numerous, and in the deepest portion of the cells the 
protoplasm becomes deeply stained, the granules very thick, as if, perhaps, 
the protoplasm were especially aggregated in the deep portion of the cell. 
The protoplasm of these epithelium cells is by no means so intensely col- 
ored as that of the blood corpuscles. This may indicate some difference in 
its constitution ; it might also indicate that the protoplasmic substance is less 
compact than in the blood-corpuscle. Very possibly both are true. We must 
note also here the even character of the protoplasm of the cells, that is, the 
absence of any non-protoplasmic matter from them, the presence of which 
- would be at once revealed by the different behaviour of the staining fluid 
toward it. 
g. The nucleus.—Between the basement membrane and the outer wall 
of the cell will be seen a row of circular bodies. These are the nuclei of the 
epithelium cells. If the section be a very thin one the nuclei will be found 
separated by a narrow space and surrounded with protoplasm. For the pres- 
ent it is enough to say that there is but one nucleus in any single cell under 
all ordinary circumstances. In studying the nucleus carefully the observer 
will note its size, somewhat larger than the blood-corpuscle (;;%)5 inch in 
diameter) ; its circular outline in the cells in question (its shape varies much 
in various tissues) ; its very sharp bounding wall ; its contents, made up of two 
sorts of substances, one which stains very deeply, and another which stains 
little or none. 
Fee 
